7.
ACKNOWLEDGMENTS
T
he author thanks the National Aeronautics and Space Administration
(NASA), the National Museums of Canada, the National Oceanic and
Atmospheric Administration (NOAA), the National Park Service, the
USDA-Soil Conservation Service, the U.S. Geological Survey (USGS), and
the Woods Hole Oceanographic Institution (WHOI) for providing photographs for this book.
Special thanks also go to Mr. Frank Darmstadt, Senior Editor at Facts On
File, and Ms. Cynthia Yazbek, Associate Editor, for their contributions in the
creation of this book.
IX

8.
foreword
INTRODUCTION TO FOSSILS AND MINERALS,
REVISED EDITION
No other aspect of geology is as popular with the public as the occurrence of
fossils and minerals in earth materials. Countless geologists have been attracted to this discipline because at an early age they discovered the wonder and
beauty of such treasures as dinosaurs and precious gems. Jon Erickson has
woven a revealing story of the uniqueness of fossils and minerals in the natural world. Indeed this attractive book provides an important source of information for those people who are interested about the world in which they
live.This introduction to fossils and minerals admirably fulfills such an objective.
When the reader goes beyond the somewhat bewildering names of fossils, there can be ample reward discovering their importance in helping decipher the mysteries of rock formations and their development throughout
time. In a somewhat different way knowledge about minerals also opens a
window to their importance. These relationships, and the manner in which
they have interacted throughout geological history, are explored in the book’s
10 chapters. The author traces Earth’s development during the millennia of
geologic time (more than 4 billion years) and shows how a knowledge of fossils and minerals is crucial for unraveling this saga.
This short book title does not adequately indicate the large diversity of
subject matter that is discussed. Indeed each chapter contains a wealth of topical information that places in perspective the essence of numerous different
XI

9.
INTRODUCTION TO FOSSILS AND MINERALS
types of material and their relationships. This systematic treatment of Earth
history, rock types, marine fossils, terrestrial fossils, crystals, gems, and precious
metals provides indelible insights into Earth’s unique character. The data are
up-to-date regarding the most recent discoveries on such topics as global tectonics and faunal extinctions, including a wealth of information about the
demise of dinosaurs.
The writing style of Jon Erickson is very clear, readable, and understandable to the nonscientist.The accuracy of the book and the scope of information will also be welcomed by geologists. The 178 figures provide visual
enhancement, and such data supplement the text descriptions. These include
maps, photographs, line drawings, and diagrams. Because the vocabulary of
geology may be new to many, a Glossary of word definitions and explanations
provides an extra bonus for comprehension. For those who are especially
interested in following the documentation of ideas and facts, the Bibliography
provides a fine summary.Thus, An Introduction to Fossils and Minerals is recommended reading for both those just beginning their discovery journey of the
Earth and for scientists who appreciate a well-crafted presentation on these
significant subjects.
—Donald R. Coates, Ph.D.
XII

10.
INTRODUCTION
he study of fossils is essential for understanding the mysteries of life, for
delineating the evolution of species, and for reconstructing the history
of the Earth. The science of geology grew out of the study of fossils,
which were used to date various strata. Today, advanced dating techniques
enable paleontologists to piece together an accurate picture of the evolution
of marine and land animals.
Interest in minerals and gems is evident in the ancient world. Crystals
continue to fascinate us with their symmetrical beauty and we depend on the
Earth’s resources for much of our energy.The rock formations in which minerals are found, reveal, layer by layer, the continual formation and erosion of
the Earth’s surface. Rocks are also known to whistle in the air, follow the sun’s
path across the sky, glow in the dark, and reverse their magnetic fields.
This revised and updated edition is a much expanded look at the popular science of paleontology and mineralogy. Readers will enjoy this clear and
easily readable text, which is well illustrated with dramatic photographs, clearly drawn illustrations, and helpful tables. The comprehensive Glossary is provided to define difficult terms, and the Bibliography lists references for further
reading.
The book is meant to introduce the fascinating science of geology and
the way it reveals the history of the Earth as told by its rocks. The text
describes the components of the Earth, the different rock types, and the methods with which the fossil and mineral contents of the rocks are found, dated,
and classified. It is also designed to aid in the location, identification, and col-
T
XIII

11.
INTRODUCTION TO FOSSILS AND MINERALS
lection of a variety of rock types, many of which contain collectible fossils and
minerals. Students of geology and science will find this a valuable reference to
further their studies.
Geologic formations can be found in most parts of the United States,
often within a short distance from home. On the basis of the information presented here, amateur geologists and collectors will have a better understanding of the forces of nature and the geologic concepts that will help them
locate rocks and minerals in the field.
XIV

13.
1
THE EARTH’S HISTORY
UNDERSTANDING OUR PLANET’S PAST
T
he Earth is a dynamic planet that is constantly changing. Continents
move about on a sea of molten rock. Jagged mountains rise to great
heights only to be eroded down to flat plains. Seas fill up and dry
out when waters are forced from the land. Glaciers expand across the land
and retreat back to the poles. And species evolve and go extinct during
times of environmental chaos and change. These episodes in the Earth’s
history are divided into chunks of time, known as the geologic time scale.
Each period of geologic history is distinct in its geologic and biologic
characteristics, and no two units of geologic time were exactly the same
(Fig. 1).
The major geologic periods were delineated by 19th-century geologists in Great Britain and western Europe (see Figure 32, p.47).The largest
divisions of the geologic record are called eras: they include the
Precambrian (time of early life), the Paleozoic (time of ancient life), the
Mesozoic (time of middle life), and the Cenozoic (time of recent life).
The eras are subdivided into smaller units called periods. Seven periods
make up the Paleozoic (though American geologists divide the
Carboniferous into Mississippian and Pennsylvanian periods), three consti1

14.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 1 The geologic
time spiral depicting the
geologic history of the
Earth.
(Earthquake Information
Bulletin 214, courtesy
USGS)
2
tute the Mesozoic, and two make up the Cenozoic. Each period is characterized by somewhat less profound changes in organisms as compared to
the eras, which mark boundaries of mass extinctions, proliferations, or rapid
transformations of species.
The two periods of the Cenozoic have been further subdivided into
seven epochs, which defined the various conditions of the period; for example, the Pleistocene witnessed a series of ice ages. The longest era, the
Precambrian, is not subdivided into individual periods as the others are,
because the poor fossil content of its rocks provides less detail about the era.
Species did not enter the fossil record in large numbers until the beginning of
the Paleozoic, about 570 million years ago, when organisms developed hard
exterior body parts, probably as a defense against fierce predators. This protection mechanism in turn gave rise to an explosion of life, which resulted in
an abundance of fossils.

15.
THE EARTH’S HISTORY
THE PRECAMBRIAN ERA
The first 4 billion years, or about nine-tenths of geologic time, constitutes the
Precambrian, the longest and least understood era of Earth history because of
the scarcity of fossils.The Precambrian is subdivided into the Hadean or Azoic
eon (time of prelife), 4.6 to 4.0 billion years ago; the Archean eon (time of initial life), 4.0 to 2.5 billion years ago; and the Proterozoic eon (time of earliest
life), 2.5 to 0.6 billion years ago. The boundary between the Archean and
Proterozoic is somewhat arbitrary and reflects major differences in the characteristics of rocks older than 2.5 billion years and those younger than 2.5 billion years. Archean rocks are products of rapid crustal formation, whereas
Proterozoic rocks represent a period of more stable geologic processes.
The Archean was a time when the Earth’s interior was hotter, the crust
was thinner and therefore more unstable, and crustal plates were more mobile.
The Earth was in a great upheaval and subjected to extensive volcanism and
meteorite bombardment, which probably had a major effect on the development of life early in the planet’s history.
About 4 billion years ago, a permanent crust began to form, composed
of a thin layer of basalt embedded with scattered blocks of granite known as
rockbergs. Ancient metamorphosed granite in the Great Slave region of
Canada’s Northwest Territories called Acasta Gneiss indicates that substantial
continental crust, comprising as much as one-fifth the present landmass, had
formed by this time. The metamorphosed marine sediments of the Isua
Formation in a remote mountainous region in southwest Greenland suggest
the presence of a saltwater ocean by at least 3.8 billion years ago.
The earliest granites combined into stable bodies of basement rock,
upon which all other rocks were deposited. The basement rocks formed the
nuclei of the continents and are presently exposed in broad, low-lying domelike structures called shields (Fig. 2). Precambrian shields are extensive uplifted areas surrounded by sediment-covered bedrock, the continental platforms,
which are broad, shallow depressions of basement complex (crystalline rock)
filled with nearly flat-lying sedimentary rocks.
Dispersed among and around the shields are greenstone belts, which
occupy the ancient cores of the continents.They comprise a jumble of metamorphosed (recrystallized) marine sediments and lava flows caught between
two colliding continents. The greenstone belts cover an area of several hundred square miles and are surrounded by immense expanses of gneiss (pronounced like the word nice), the metamorphic equivalent of granite and the
predominant Archean rock type. The rocks are tinted green as a result of the
presence of the mineral chlorite and are among the best evidence for plate
tectonics (Fig. 3), the shifting of crustal plates on the Earth’s surface, early in
the Precambrian.
3

16.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 2 The location of
Precambrian continental
shields, the nuclei upon
which the continents grew,
which comprise the oldest
rocks on Earth.
Geologists are particularly interested in greenstone belts because they not
only provide important evidence for plate tectonics but also contain most of the
world’s gold. India’s Kolar greenstone belt holds the richest gold deposits. It is
some 3 miles wide and 50 miles long and formed when two plates clashed about
2.5 billion years ago. In Africa, the best deposits are in rocks as old as 3.4 billion
years, and most South African gold mines are found in greenstone belts. In North
America, the best gold mines are in the greenstone belts of the Great Slave region
of northwestern Canada, where well over 1,000 deposits are known.
The greenstone belts also comprised ophiolites, from the Greek ophis,
meaning “serpent.” They are slices of ocean floor shoved up onto the contiFigure 3 The plate tectonics model, in which
new oceanic crust is generated at spreading ridges
and old oceanic crust is
destroyed in subduction
zones, or trenches, along
the edges of continents or
island arcs, processes that
move the continents
around the face of the
Earth.
Ridge
Trench
Crust
re
osphe
Lith
Magma
Magma
Mantle
4

17.
THE EARTH’S HISTORY
nents by drifting plates and are as much as 3.6 billion years old. In addition, a
number of ophiolites contain ore-bearing rocks that are important mineral
resources the world over. Pillow lavas, which are tubular bodies of basalt
extruded undersea, appear in the greenstone belts as well, signifying that the
volcanic eruptions took place on the ocean floor. Because greenstone belts are
essentially Archean in age, their disappearance from the geologic record
around 2.5 billion years ago marks the end of the Archean eon.
The Proterozoic eon, 2.5 to 0.6 billion years ago, was a shift to calmer
times as the Earth matured from adolescence to adulthood. When the eon
began, as much as 75 percent of the current continental crust had formed.
Continents were more stable and welded together into a single large supercontinent. Extensive volcanic activity, magmatic intrusions, and rifting and
patching of the crust built up the continental interiors, while erosion and sedimentation built the continental margins outward. The global climate of the
Proterozoic was significantly cooler, and the Earth experienced its first major
ice age between 2.3 and 2.4 billion years ago (Table 1).
By the beginning of the Proterozoic, most of the material that is now
locked up in sedimentary rocks was at or near the surface. In addition, ample
sources of Archean rocks were available for erosion and redeposition into
TABLE 1
CHRONOLOGY OF THE MAJOR ICE AGES
Time (years)
Event
10,000–present
15,000–10,000
20,000–18,000
100,000
1 million
3 million
4 million
15 million
30 million
65 million
250–65 million
250 million
700 million
2.4 billion
Present interglacial
Melting of ice sheets
Last glacial maximum
Most recent glacial episode
First major interglacial
First glacial episode in Northern Hemisphere
Ice covers Greenland and the Arctic Ocean
Second major glacial episode in Antarctica
First major glacial episode in Antarctica
Climate deteriorates; poles become much colder
Interval of warm and relatively uniform climate
The great Permian ice age
The great Precambrian ice age
First major ice age
5

18.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 4 Glacial landscape high on the south
flank of Uinta
Mountains, Duchesne
County, Utah. An
unnamed ice-sculptured
peak at the head of Rock
Creek Basin looms above
a morainal ridge in the
foreground.
(Photo by W. R. Hansen,
courtesy USGS)
Proterozoic rock types. Sediments derived directly from primary sources are
called wackes, often described as dirty sandstone. Most Proterozoic wackes
composed of sandstones and siltstones originated from Archean greenstones.
Another common rock type was a fine-grained metamorphosed rock called
quartzite, derived from the erosion of siliceous grainy rocks such as granite
and arkose, a coarse-grain sandstone with abundant feldspar.
Conglomerates, which are consolidated equivalents of gravels, were also
abundant in the Proterozoic. Nearly 20,000 feet of Proterozoic sediments lie
in the Uinta Range of Utah (Fig. 4), one of the only two major east-west
trending mountain ranges in North America. The Montana Proterozoic belt
system contains sediments over 11 miles thick.The Proterozoic is also known
for its terrestrial redbeds, composed of sandstones and shales cemented by iron
oxide, which colored the rocks red. Their appearance, around 1 billion years
ago, indicates that the atmosphere contained substantial amounts of oxygen,
which oxidized the iron in a process similar to the rusting of steel.
The weathering of primary, or parental, rocks during the Proterozoic
also produced solutions of calcium carbonate, magnesium carbonate, calcium
sulfate, and sodium chloride, which in turn precipitated into limestone,
dolomite, gypsum (see Glossary), and halite (rock salt). The Mackenzie
Mountains of northwestern Canada contain dolomite deposits more than a
mile thick.These minerals are thought to be mainly chemical precipitates and
not of biologic origin. Carbonate rocks, such as limestone and chalk, produced
chiefly by the deterioration of shells and skeletons of simple organisms,
6

19.
THE EARTH’S HISTORY
became much more common during the latter part of the Proterozoic,
between about 700 and 570 million years ago, whereas during the Archean,
they were relatively rare because of the scarcity of lime-secreting organisms.
During the Proterozoic, the continents were composed of odds and ends
of Archean cratons, which are ancient, stable rocks in continental interiors.
The original cratons formed within the first 1.5 billion years and totaled
about one-tenth of the present landmass. They numbered in the dozens and
ranged in size from about a fifth the area of today’s North America to smaller than the state of Texas.The cratons are composed of highly altered granite
and metamorphosed marine sediments and lava flows. The rocks originated
from intrusions of magma into the primitive oceanic crust.
Several cratons welded together to form an ancestral North American
continent called Laurentia (Fig. 5). Most of the continent, comprising the
interior of North America, Greenland, and northern Europe, evolved in a relatively brief period of only 150 million years. Laurentia continued to grow by
garnering bits and pieces of continents and chains of young volcanic islands.
A major part of the continental crust underlying the United States from
Arizona to the Great Lakes to Alabama formed in one great surge of crustal
generation around 1.8 billion years ago that has no equal.This buildup possibly resulted from greater tectonic activity and crustal generation during the
Proterozoic than during any subsequent time of Earth history.
After the rapid continent building, the interior of Laurentia experienced
extensive igneous activity that lasted from 1.6 to 1.3 billion years ago.A broad
belt of red granites and rhyolites, which are igneous rocks formed by solidifying of molten magma below ground as well as on the surface, extended sev-
Figure 5 The cratons
that constitute North
America came together
some 2 billion years ago.
Slave craton
North Atlantic craton
Northwest Churchill craton
Superior craton
Wyoming craton
Penokean Orogen
Grenville Province
7

20.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 6 Late
Precambrian Ediacara
fauna from Australia.
8
eral thousand miles across the interior of the continent from southern
California to Labrador. The Laurentian granites and rhyolites are unique
because of their sheer volume, which suggests that the continent stretched and
thinned almost to the breaking point.
These rocks are presently exposed in Missouri, Oklahoma, and a few
other localities but are buried under sediments up to a mile thick in the center of the continent. In addition, vast quantities of molten basalt poured from
a huge tear in the crust running from southeast Nebraska into the Lake
Superior region about 1.1 billion years ago. Arcs of volcanic rock also weave
through central and eastern Canada down into the Dakotas.
Marine life during the Proterozoic was highly distinct from that of the
Archean and represented considerable biologic advancement. Numerous
impressions of strange extinct species have been found in the Ediacara
Formation of southern Australia, dated around 670 million years old (Fig. 6).
This great diversity of species followed the Precambrian ice age, the most
extensive glaciation on Earth, when nearly half the land surface was covered
with glaciers.When the ice retreated and the seas began to warm, life took off
in all directions.
Unique and bizarre creatures preserved in Australia’s Ediacara Formation
thrived in the ocean, and a greater percentage of experimental organisms, animals that evolved unusual characteristics, came into being at that time than
during any other interval of Earth history. As many as 100 phyla, organisms
that shared similar body styles, existed, whereas only about a third as many
phyla are living today. This biologic exuberance set the stage for the

21.
the earth’s history
Phanerozoic eon, or the time of later life, comprising the Paleozoic, Mesozoic,
and Cenozoic eras. For the first time, fossilized remains of animals became
abundant, because of the evolution of lime-secreting organisms that constructed hard shells in the lower Paleozoic.
Toward the end of the Proterozoic, between 630 and 560 million years
ago, a supercontinent named Rodinia, Russian for “motherland,” located near
the equator, rifted apart into four or five major continents, although they were
configured much differently than they are today. The breakup produced
extensive continental margins, where vast carbonate belts formed. This
extended shoreline provided additional habitat area, which along with warm
Cambrian seas might have played a major role in the rapid explosion of new
species by the start of the Paleozoic.
THE PALEOZOIC ERA
The Paleozoic era, which spans a period from about 570 to about 250 million
years ago, was a time of intense growth and competition in the ocean and later
on the land, culminating with widely dispersed and diversified species. By the
middle of the era, all major animal and plant phyla were already in existence.
The earliest period of the Paleozoic is called the Cambrian, named for the
Cambrian mountain range of central Wales, where sediments containing the
earliest known fossils were found. Thus, the base of the Cambrian was once
thought to be the beginning of life, and all previous time was known as the
Precambrian.
The Paleozoic is generally divided into two time units of nearly equal
duration. The lower Paleozoic consists of the Cambrian, Ordovician, and
Silurian periods, and the upper Paleozoic comprises the Devonian,
Carboniferous, and Permian periods. The first half of the Paleozoic was relatively quiet in terms of geologic processes, with little mountain building, volcanic activity, or glaciation and no extremes in climate. Most of the continents
were located near the equator; that location explains the presence of warm
Cambrian seas. Sea levels rose and flooded large portions of the land. The
extended shoreline might have spurred the explosion of new species, producing twice as many phyla during the Cambrian as before or since. Never were
so many experimental organisms in existence, none of which has any modern
counterparts. Most new species of the early Cambrian were short-lived, however, and became extinct.
During the late Precambrian and early Cambrian, a proto–Atlantic
Ocean called the Iapetus opened, forming extensive inland seas.The inundation submerged most of Laurentia and the ancient European continent called
Baltica.The Iapetus Sea was similar in size to the North Atlantic and occupied
9

22.
INTRODUCTION TO FOSSILS AND MINERALS
the same general location about 500 million years ago. It was dotted with volcanic islands, resembling the present-day southwestern Pacific Ocean. The
shallow waters of the near-shore environment of this ancient sea contained
abundant invertebrates, including trilobites, which accounted for about 70
percent of all species.
During the Cambrian, continental motions assembled the present continents of Africa, South America,Australia,Antarctica, and India into a large landmass called Gondwana (Fig. 7), named for an ancient region of east-central India.
Much of Gondwana was in the South Polar region from the Cambrian to the
Silurian. The present continent of Australia sat on the equator at the northern
edge of Gondwana. A major mountain building episode from the Cambrian to
the middle Ordovician deformed areas between all continents comprising
Gondwana, indicating their collision during this interval. Extensive igneous
activity and metamorphism accompanied the mountain building at its climax.
During the late Silurian, Laurentia collided with Baltica and closed off
the Iapetus. The collision fused the two continents into Laurasia, named for
the Laurentian province of Canada and the Eurasian continent, about 400 million years ago. These Paleozoic continental collisions raised huge masses of
rocks into several mountain belts throughout the world. The sutures joining
the landmasses are preserved as eroded cores of ancient mountains called orogens from the Greek word oros, meaning “mountain.”
Figure 7 The configuration of the southern continents that comprised
Gondwana.
Africa
India
South America
Antarctica
10
Australia

23.
the earth’s history
Laurasia and Gondwana were separated by a large body of water, the
Tethys Sea, named for the mother of the seas in Greek mythology. Thick
deposits of sediments washed off the continents flowed into the Tethys and
were later squeezed by continental collisions and uplifted into mountain belts.
The continents were lowered by erosion, and shallow seas flowed inland,
flooding more than half the landmass. The inland seas and wide continental
margins along with a stable environment provided excellent growing conditions for marine life to flourish and migrate throughout the world.
The widespread distribution of evaporite deposits in the Northern
Hemisphere, coal deposits in the Canadian Arctic, and carbonate reefs suggest
a warm climate and desert conditions over large areas.Warm temperatures of
the past are generally indicated by abundant marine limestones, dolostones,
and calcareous shales. A coal belt, extending from northeastern Alaska across
the Canadian archipelago to northernmost Russia, suggests that vast swamps
were prevalent in these regions.The ideal climate setting helped spur the rise
of the amphibians that inhabited the great Carboniferous swamps.
The second half of the Paleozoic followed on the heels of a Silurian ice
age, when Gondwana wandered into the South Polar region around 400 million years ago and acquired a thick sheet of ice. As the seas lowered and the
continents rose, the inland seas departed and were replaced by great swamps.
In these regions, vast coal deposits accumulated during the Carboniferous,
which had the highest organic burial rates of any period in Earth history.
Extensive forests and swamps grew successively on top of one another and
continued to add to thick deposits of peat, which were buried under layers of
sediment and compressed into lignite, bituminous, and anthracite coal.
Beginning in the late Devonian and continuing into the Carboniferous,
Gondwana and Laurasia converged into the supercontinent Pangaea, Greek
for “all lands,” which comprised some 40 percent of the Earth’s total surface
area and extended practically from pole to pole. A single great ocean called
Panthalassa, Greek for “universal sea,” stretched uninterrupted across the rest
of the planet. Over the ensuing time, smaller parcels of land continued to collide with the supercontinent until it peaked in size by the beginning of the
Triassic, about 210 million years ago.
The closing of the Tethys Sea during the assembly of Pangaea eliminated a major barrier to the migration of species from one continent to another, allowing them to disperse to all parts of the world. Plant and animal life
witnessed a great diversity in the ocean as well as on land. A continuous
shallow-water margin extended around the entire perimeter of Pangaea, with
no major physical barriers to hamper the dispersal of marine life.The formation of Pangaea spurred a great proliferation of plant and animal life and
marked a major turning point in evolution of species, during which the reptiles emerged as the dominant land animals.
11

24.
INTRODUCTION TO FOSSILS AND MINERALS
The Pangaean climate was one of extremes, with the northern and
southern regions as cold as the Arctic and the interior as hot as a desert, where
almost nothing grew. The massing of continents together created an overall
climate that was hotter, drier, and more seasonal than at any other time in geologic history. As the continents rose higher and the ocean basins dropped
lower, the land became dryer and the climate grew colder, especially in the
southernmost lands, which were covered with glacial ice. Continental margins
were less extensive and were narrower, placing severe restrictions on marine
habitat. By the close of the Paleozoic, the southern continents were in the
grips of a major ice age.
During the Permian, all the interior seas retreated from the land, as an
abundance of terrestrial redbeds and large deposits of gypsum and salt were
laid down. Extensive mountain building raised massive chunks of crust.A continuous, narrow continental margin surrounded the supercontinent, reducing
the shoreline, thus radically limiting the marine habitat area. Moreover, unstable near-shore conditions resulted in an unreliable food supply. Many species
unable to cope with the limited living space and food supply died out in tragically large numbers. The extinction was particularly devastating to Permian
marine fauna. Half the families of aquatic organisms, 75 percent of the
amphibian families, and over 80 percent of the reptilian families, representing
more than 95 percent of all known species, abruptly disappeared. In effect, the
extinction left the world almost as devoid of species at the end of Paleozoic as
at the beginning.
THE MESOZOIC ERA
The Mesozoic era, from about 250 to about 65 million years ago, comprises the
Triassic, Jurassic, and Cretaceous periods. When the era began, the Earth was
recovering from a major ice age and the worst extinction event in geologic history.Thus, the bottom of the Mesozoic was a sort of rebirth of life, and 450 new
families of organisms came into existence. However, instead of developing
entirely new body styles, as in the early Paleozoic, the start of the Mesozoic saw
only new variations on already established themes.Therefore, fewer experimental organisms evolved, and many of the lines of today’s species came into being.
At the beginning of the era, all the continents were consolidated into a
supercontinent, about midway they began to break up, and at the end they
were well along the path to their present locations (Fig. 8). The breakup of
Pangaea created three major bodies of water, the Atlantic, Arctic, and Indian
Oceans.The climate was exceptionally mild for an unusually long period, possibly as a result of increased volcanic activity and the resultant greenhouse
effect. One group of animals that excelled during these extraordinary condi12

25.
the earth’s history
Figure 8 The breakup
and drift of the continents.
L L a uR AsS I A
AU ra ia
Pangaea
Tethys
Sea
G G ondw ana A
ONDWA N
225 million years ago
180 million years ago
135 million years ago
65 million years ago
tions were the reptiles. Some reptilian species returned to the sea; others took
to the air.They occupied nearly every corner of the globe; that is why the era
is generally known as the “age of the reptiles.”
In the early Triassic, the great glaciers of the previous ice age melted, and
the seas began to warm. The energetic climate facilitated the erosion of the
high mountain ranges of North America and Europe. Seas retreated from the
continents as they continued to rise, and widespread deserts covered the land.
Abundant terrestrial redbeds and thick beds of gypsum and salt were deposited in the abandoned basins. A preponderance of red rocks composed of sandstones and shales are exposed in the mountains and canyons in the western
United States (Fig. 9). Terrestrial redbeds covered a region from Nova Scotia
to South Carolina and the Colorado Plateau. Redbeds were also common in
Europe, especially in northwestern England.
13

26.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 9 A redbed formation on the east side of
the Bighorn Mountains,
Johnson County,
Wyoming.
(Photo by N. H. Darton,
courtesy USGS)
Huge lava flows and granitic intrusions invaded Siberia, and extensive
lava flows covered South America, Africa, and Antarctica as well. In South
America, great floods of basalt, upward of 2,000 feet or more thick, blanketed
large parts of Brazil and Argentina. Triassic basalts in eastern North America
erupted from a great rift that separated the continent from Eurasia. Basalt
flows also envelop a region from Alaska to California. These large volcanic
eruptions created a series of overlapping lava flows, giving many exposures a
terracelike appearance known as traps, from the Dutch word for “stairs.”
Early in the Jurassic period, North America separated from South
America, and a great rift divided the North American and Eurasian continents.
The rupture separating the continents flooded with seawater to form the infant
North Atlantic Ocean. India, nestled between Africa and Antarctica, drifted away
from Gondwana, and Antarctica, still attached to Australia, swung away from
Africa to the southeast, forming the proto–Indian Ocean. During the Jurassic
and continuing into the Cretaceous, an interior sea flowed into the west-central
portions of North America. Massive accumulations of marine sediments eroded
from the Cordilleran highlands to the west (sometimes referred to as the ancestral Rockies) were deposited on the terrestrial redbeds of the Colorado Plateau,
forming the Jurassic Morrison Formation, well known for fossil bones of large
dinosaurs (Fig. 10). Eastern Mexico, southern Texas, and Louisiana were also
flooded, and seas inundated South America, Africa, and Australia as well.
The continents were flatter, mountain ranges were lower, and sea levels
were higher.Thick deposits of sediment that filled the inland marine basins of
North America were uplifted and eroded, providing the western United States
with its impressive landscapes. Reef building was intense in the Tethys Sea, and
thick deposits of limestone and dolomite were laid down by lime-secreting
14

27.
the earth’s history
organisms in the interior seas of Europe and Asia. These deposits were later
uplifted during one of geologic history’s greatest mountain building episodes.
The rim of the Pacific Basin became a hotbed of geologic activity, and practically all mountain ranges facing the Pacific Ocean and island arcs along its
perimeter developed during this period.
During the Cretaceous period, plants and animals were especially prolific and ranged practically from pole to pole. Huge deposits of limestone and
chalk created in Europe and Asia gave the period its name, from the Latin creta,
meaning “chalk.” Mountains were lower and sea levels higher, and the total
land surface declined to perhaps half its present size.
In the late Cretaceous and early Tertiary, land areas were inundated by the
ocean, which flooded continental margins and formed great inland seas, some
of which split continents in two. Seas divided North America in the Rocky
Mountain and high plains regions, South America was cut in two in the region
that later became the Amazon Basin, and Eurasia was split by the joining of the
Tethys Sea and the newly formed Arctic Ocean.The oceans of the Cretaceous
were also interconnected in the equatorial regions by the Tethys and Central
American seaways, providing a unique circumglobal oceanic current system
that made the climate equable, with no extremes in weather.
Toward the end of the Cretaceous, North America and Europe were no
longer in contact except for a land bridge created by Greenland to the north.
The Bering Strait between Alaska and Asia narrowed, creating the Arctic
Figure 10 A dinosaur
boneyard at the Howe
Ranch quarry near
Cloverly,Wyoming.The
dinosaurs, along with
70 percent of all other
known species, abruptly
went extinct 65 million
years ago.
(Photo by N. H. Darton,
courtesy USGS)
15

28.
INTRODUCTION TO FOSSILS AND MINERALS
Ocean, which was practically land-locked.Africa moved northward and began
to close the Tethys Sea, leaving Antarctica, which was still attached to Australia,
far behind. As Antarctica and Australia moved eastward, a rift developed and
began to separate them.
Meanwhile, India began to cross the equator and narrow the gap separating it from southern Asia.The crust rifted open on the west side of India,
and massive amounts of molten rock poured onto the landmass, blanketing
much of west central India, known as the Deccan Traps. Over a period of several million years about 100 individual basalt flows produced over 350,000
cubic miles of lava, up to 8,000 feet thick. Continental rifting during the same
time began separating Greenland from Norway and North America.The rifting poured out great flood basalts across eastern Greenland, northwestern
Britain, northern Ireland, and the Faeroe Islands between Britain and Iceland.
When the Cretaceous came to an end, the seas receded from the land as
sea levels lowered and the climate grew colder.The decreasing global temperatures and increasing seasonal variation in the weather made the world more
stormy, with powerful gusty winds that wreaked havoc over the Earth. These
conditions might have had a major impact on the climatic and ecologic stability of the planet, possibly leading to the great extinction at the end of the era.
THE CENOZOIC ERA
The Cenozoic era, from about 65 million years ago to the present, comprises
the Tertiary period, which occupies most of the era, and the Quaternary period, which covers the last 2 million years. Both terms were adapted from the old
geologic time scale in which the Primary and Secondary periods represented
ancient Earth history. The pronounced unequal lengths of the two periods
acknowledge a unique sequence of ice ages during the Pleistocene epoch.
Most European and many American geologists prefer to subdivide the
Cenozoic into two nearly equal time intervals.The first is the Paleogene period, from about 65 to about 26 million years ago, which includes the
Paleocene, Eocene, and Oligocene epochs. The second is the Neogene period, from about 26 million years ago to the present, which includes the
Miocene, Pliocene, Pleistocene, and Holocene epochs. Whichever time scale
is used, the Cenozoic is generally regarded as the “age of mammals.”
The Cenozoic was a time of constant change, as all species had to adapt
to a wide range of living conditions. Changing climate patterns resulted from
the movement of continents toward their present positions, and intense tectonic activity built a large variety of landforms and raised most mountain ranges of
the world. Except for a few land bridges exposed from time to time, plants and
animals were prevented from migrating from one continent to another.
16

29.
the earth’s history
About 57 million years ago, Greenland began to separate from North
America and Eurasia. Prior to about 4 million years ago, Greenland was largely ice-free, but today the world’s largest island is buried under a sheet of ice
up to two miles thick.At times,Alaska connected with east Siberia to close off
the Arctic Basin from warm water currents originating from the tropics,
resulting in the formation of pack ice in the Arctic Ocean.
Antarctica and Australia broke away from South America and moved
eastward. The two continents then rifted apart, with Antarctica moving
toward the South Pole and Australia continuing in a northeastward direction.
In the Eocene, about 40 million years ago, Antarctica drifted over the South
Pole and acquired a permanent ice sheet that buried most of its terrain features (Fig. 11).
The Cenozoic is also known for its intense mountain building, when
highly active tectonic forces established the geological provinces of the western United States (Fig. 12). The Rocky Mountains, extending from Mexico
to Canada, heaved upward during the Laramide orogeny (mountain-building
episode) from about 80 million to 40 million years ago.A large number of parallel faults sliced through the Basin and Range Province, between the Sierra
Nevada and the Wasatch Mountains, during the Oligocene, producing parallel, north-south–trending mountain ranges. During the last 10 million years,
California’s Sierra Nevada rose about 7,000 feet.
Figure 11 Taylor Glacier
region,Victoria Land,
Antarctica.
(Photo by W. B. Hamilton,
courtesy USGS)
17

30.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 12 Geologic
provinces of the western
United States.
Idaho
Batholith
Ro
ck
Columbia
Plateau
y
Mo
un
tain
Basin
s
and
t
ev ada
S i err a N
Pacific Coas
Colorado
Great Plains
Plateau
Ra
ng
e
About 50 million years ago, the collision of the African plate with the
Eurasian plate squeezed out the Tethys, creating a long chain of mountains and
two major inland seas, the ancestral Mediterranean and a composite of the Black,
Caspian, and Aral Seas, called the Paratethys, that covered much of eastern
Europe.Thick sediments that had been accumulating for tens of millions of years
on the bottom of the Tethys buckled into long belts of mountain ranges on the
northern and southern flanks. This episode of mountain building, called the
Alpine orogeny, ended around 26 million years ago and marks the boundary
between the Paleogene and Neogene periods.The Alps of northern Italy formed
when the Italian prong of the African plate thrust into the European plate.
The collision of India with southern Asia, around 45 million years ago,
uplifted the tall Himalaya Mountains and the broad three-mile-high Tibetan
Plateau, whose equal has not existed on this planet for more than a billion
years. The mountainous spine that runs along the western edge of South
America forming the Andes Mountains continued to rise throughout more
than of the Cenozoic as a result of the subduction of the Nazca plate beneath
the South American plate (Fig. 13). The melting of the subducting plate fed
18

31.
the earth’s history
magma chambers (volcanic reservoirs) with molten rock, causing numerous
volcanoes to erupt in one fiery outburst after another.
Volcanic activity was extensive throughout the world during the
Tertiary, whose strong greenhouse effect might explain in part why the Earth
grew so warm during the Eocene epoch from 54 million to 37 million years
ago. A band of volcanoes stretching from Colorado to Nevada produced a
series of very violent eruptions between 30 million and 26 million years ago.
The massive outpourings of carbon dioxide–laden lava might have created the
extraordinary warm climate that sparked the evolution of the mammals.
Winters were warm enough for crocodiles to roam as far north as Wyoming,
and forests of palms, cycads, and ferns covered Montana.
Crustal movements in the Oligocene, about 25 million years ago,
brought about changes in relative motions between the North American
plate and the Pacific plate, creating the San Andreas Fault system running
through southern California (Fig. 14). Baja California split off from North
America and opened up the Gulf of California. This provided a new outlet to the sea for the Colorado River, which began to carve out the Grand
Canyon.
Beginning about 17 million years ago and extending for a period of 2
million years, great outpourings of basalt covered Washington, Oregon, and
Idaho, creating the Columbia River Plateau (Fig. 15). Massive floods of lava
enveloped an area of about 200,000 square miles, in places reaching 10,000
feet thick.The tall volcanoes of the Cascade Range from northern California
to Canada erupted in one great profusion after another. Extensive volcanism
Figure 13 The lithospheric plates that constitute the Earth’s crust.
Note the position of
Nazca and the South
American plates.
(Courtesy USGS)
19

32.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 14 The San
Andreas Fault in southern
California.
(Photo by R. E.Wallace,
courtesy USGS)
Figure 15 Palouse Falls
in Columbia River
basalt, FranklinWhitman Counties,
Washington.
(Photo by F. O. Jones, courtesy USGS)
20
occurred in the Colorado Plateau and Sierra Madre regions as well. Iceland is
an expression of the Mid-Atlantic Ridge, where massive floods of basalt 16
million years ago formed a huge volcanic plateau 900 miles wide, over onethird of which rose above sea level.
About 3 million years ago, the Panama Isthmus separating North and
South America uplifted as oceanic plates collided, precipitating a lively
exchange of species between the continents. The new landform halted the
flow of cold water currents from the Atlantic into the Pacific, which along
with the closing of the Arctic Ocean from warm Pacific currents might have
initiated the Pleistocene glaciation. Never before has permanent ice existed at
both poles, suggesting that the planet has been steadily cooling since the
Cretaceous. By the time the continents had wandered to their present positions and the mountain ranges had attained their current elevations, the world
was ripe for the coming of the ice age.

33.
the earth’s histo-
Figure 16 The extent of
glaciation during the last
ice age.
THE PLEISTOCENE ICE AGE
The Pleistocene epoch witnessed a progression of ice ages advancing and retreating almost by clockwork. About 3 million years ago, huge volcanic eruptions in
the northern Pacific darkened the skies, and global temperatures plummeted, culminating in a series of glacial episodes. During the last ice age, massive ice sheets
swept out of the polar regions, and glaciers up to two miles or more thick
enveloped Canada, Greenland, and northern Eurasia (Fig. 16).The glaciers covered some 11 million square miles of land that is presently ice-free.The glaciation began with a rapid buildup of glacial ice some 115,000 years ago, intensified
about 75,000 years ago, and peaked about 18,000 years ago.
North America was engulfed by two main glacial centers. The largest
glacier, called the Laurentide, blanketed an area of about 5 million square
miles. It extended from Hudson Bay and reached northward into the Arctic
Ocean and southward into eastern Canada, New England, and the upper midwestern United States. A smaller ice sheet, called the Cordilleran, originated
in the Canadian Rockies and enveloped western Canada and the northern
and southern sections of Alaska, leaving an ice-free corridor down the center
of the present state. Scattered glaciers also covered the mountainous regions of
the northwestern United States. Ice buried the mountains of Wyoming,
Colorado, and California, and rivers of ice linked the North American
cordillera with mountains in Mexico.
Europe was engulfed by two major ice sheets as well. The largest, the
Fennoscandian, fanned out from northern Scandinavia and covered most of
Great Britain as far south as London and large parts of northern Germany,
21

34.
INTRODUCTION TO FOSSILS AND MINERALS
Poland, and European Russia.A smaller ice sheet, known as the Alpine and centered in the Swiss Alps, enveloped parts of Austria, Italy, France, and southern
Germany. In Asia, glaciers occupied the Himalayas and blanketed parts of Siberia.
In the Southern Hemisphere, only Antarctica held a major ice sheet,
which expanded to about 10 percent larger than its present size and extended as far as the tip of South America. Sea ice surrounding Antarctica nearly
doubled its modern wintertime area. Smaller glaciers capped the mountains of
Australia, New Zealand, and the Andes of South America, the latter of which
contained the largest of the southern alpine ice sheets.Throughout the rest of
the world, mountain glaciers topped peaks that are currently ice-free.
The lower temperatures reduced the evaporation rate of seawater and
decreased the average amount of precipitation, causing expansion of deserts in
many parts of the world. The fierce desert winds produced tremendous dust
storms, and the dense dust suspended in the atmosphere blocked sunlight,
keeping temperatures well below present-day averages. Most of the windblown sand deposits called loess in the central United States were laid down
during the Pleistocene ice ages.
Approximately 5 percent of the planet’s water was locked up in glacial
ice.The continental ice sheets contained approximately 10 million cubic miles
of water and covered about one-third the land surface with glacial ice three
times its current size. The accumulated ice dropped sea levels about 400 feet
and shorelines advanced seaward up to 100 miles or more.The drop in sea level
exposed land bridges and linked continents, spurring a vigorous migration of
species, including humans, to various parts of the world.Adaptations to the cold
climate allowed certain species of mammals to thrive in the ice-free regions of
the northern lands. Giant mammals, including the mammoth, sabertooth cat,
and giant sloth, roamed many parts of the Northern Hemisphere that were free
of glaciers.
Perhaps one of the most dramatic climate changes in geologic history
took place during the present interglacial known as the Holocene epoch,
which began about 11,000 years ago. After some 100,000 years of gradual
accumulation of snow and ice up to two miles and more thick, the glaciers
melted away in only a matter of several thousand years, retreating several hundred feet annually.The retreating glaciers left an assortment of glacial deposits
in their wake, including sinuous eskers, elongated drumlins, and immense
boulder fields (Fig. 17). About a third of the ice melted between 16,000 and
12,000 years ago, when average global temperatures increased about five
degrees Celsius to nearly present levels. A renewal of the deep-ocean circulation system, which was shut off or weakened during the ice age, might have
thawed out the planet from its deep freeze.
The demise of the giant ice sheets and the subsequent warming of the
climate left many puzzles such as an unusual occurrence of hippopotamus
22

35.
the earth’s history
Figure 17 A perched
erratic boulder left by the
ice of the El Portal glaciation, near the head of
Little Cottonwood Creek,
east of Army Pass, Inyo
County, California.
(Photo by F. E. Mathes,
courtesy USGS)
bones in the deserts of Africa. During a wet period between 12,000 and 6,000
years ago, some of today’s African deserts were covered with large lakes. Lake
Chad, lying on the border of the Sahara Desert, appears to have swelled over
10 times its present size. Swamps, long since vanished, once harbored large
populations of hippopotamuses and crocodiles, whose fossil bones now bake
in the desert sands.
After sampling a little geologic history, the next chapter shows how fossils helped uncover clues to the past.
23

36.
2
CLUES TO THE PAST
THE PRINCIPLES OF PALEONTOLOGY
F
ossils have been known from ancient times, and perhaps the first to
speculate on their origin were the early Greeks. The Greek philosophers recognized that seashells found in the mountains were the
remains of once-living creatures. Although Aristotle clearly recognized that
certain fossils such as fish bones were the remains of organisms, he generally
believed that fossils were placed in the rocks by a celestial influence. This
astrologic account for fossils maintained its popularity throughout the Middle
Ages. During this time, competing fossil theories included the idea that fossils grew in rocks, were discarded creations, or were tricks of the devil to
deceive humans about the true history of the world. Fossils were also thought
to be the creations of Mother Nature in a playful mood.
Not until the Renaissance period and the rebirth of science did people
pursue alternate explanations for the existence of fossils that were based on
scientific principles. By the 1700s, most scientists began to accept fossils as the
remains of organisms because they closely resembled living things rather than
merely inorganic substances such as concretions or nodules in rock. When
placed in their proper order, fossils pieced together a nearly complete historical account of life on Earth, showing clear evidence for the evolution and
extinction of species.
24

37.
clues to the past
KEYS TO THE HISTORY OF LIFE
One of the major problems encountered when exploring for fossils of early
life is that the Earth’s crust is constantly rearranging itself, and only a few
fossil-bearing formations have survived undisturbed over time, the others having been eroded away. Therefore, the history of the Earth as told by its fossil
record is not completely known because of the remaking of the surface, which
erases whole chapters of geologic history.Yet the study of fossils along with
the radiometric dating of the rocks that contain them have constructed a reasonably good chronology of Earth history.
Bacteria, which descended from the earliest known form of life, remain
by far the most abundant organisms, and without them no other life-forms
could exist. Evidence that life began very early in the Earth’s history when the
planet was still quite hot exists today in the form of thermophilic (heat-loving)
bacteria, found in thermal springs and other hot-water environments (Fig. 18).
Because these bacteria lack a nucleus, which ceases to function in hot water,
they can live and reproduce successfully even at temperatures well above the
normal boiling point of water as long as it remains a liquid, which requires
pressures equal to those in the deep sea. The existence of these organisms is
used as evidence that thermophiles were the ancestors of all life on Earth.
Life probably had a very difficult time at first. When living organisms
began evolving, the Earth was constantly bombarded with large meteorites.As
a result, the first living forms might have been repeatedly killed off, forcing life
to regenerate over and over again. Whenever primitive organic molecules
began to be arranged into living cells, the gigantic impacts would have blasted them apart before they could reproduce. One safe place where life would
Figure 18 Boiling mud
springs northwest of
Imperial Junction,
California.
(Photo by W. C.
Mendenhall, courtesy
USGS)
25

38.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 19 Tall tube
worms, giant clams, and
large crabs occupy the
seafloor near the
hydrothermal vents.
be free to evolve was the bottom of the ocean, where hydrothermal vents provided warmth and nourishment.Today, these areas contain some of the most
bizarre creatures the Earth has ever known (Fig. 19).
Among the oldest fossils found on Earth are the remains of ancient
microorganisms and stromatolites (Fig. 20), layered structures formed by the
accretion of fine sediment grains by matted colonies of cyanobacteria (formerly called blue-green algae).These were found in 3.5-billion-year-old sedimentary rocks of the Warrawoona group in a desolate place called North Pole
in Western Australia.Associated with these rocks were cherts (hard rocks composed of microscopic crystals of silica) with microfilaments, which are small,
threadlike structures, possibly of bacterial origin.
Most Precambrian cherts are thought to be chemical sediments precipitated from silica-rich water in a deep ocean. The abundance of chert in the
early Precambrian is evidence that most of the Earth’s crust was deeply submerged in a global ocean during that time. However, cherts at the North Pole
site appear to have had a shallow-water origin.This silica probably leached out
of volcanic rocks that erupted into shallow seas.The silica-rich water circulated through porous sediments, dissolving the original minerals and precipitating silica in their place. Microorganisms buried in the sediments were encased
26

39.
clues to the past
Figure 20 Stromatolite
beds from a cliff above the
Regal mine, Gila County,
Arizona.
(Photo by A. F. Shride,
courtesy USGS)
in one of the hardest natural substances and thus were able to withstand the
rigors of time.
Similar cherts with microfossils of filamentous bacteria dating 3.4 billion
years old have been found in eastern Transvaal, South Africa. In addition, 2billion-year-old cherts from the Gunflint iron formation on the north shore
of Lake Superior contained similar microfossils. These rocks were originally
mined for flint to fire the flintlock rifles of the early settlers until the discovery there of iron, which made this region one of the best iron mining districts
in the country.
About 500 million years after the formation of the Gunflint chert, a new
type of cell, called a eukaryote, emerged in the fossil record. It was character27

40.
INTRODUCTION TO FOSSILS AND MINERALS
ized by a nucleus that allowed chromosomes to divide and unite hereditary
material in a systematic manner. A greater number of genetic mutations were
produced, providing a wide variety of organisms, some of which might have
adapted to their environment better than others. These organisms were the
forerunners of all the complex forms of life on Earth today.
By far, the most numerous fossils representing the first abundant life on
Earth were the hard parts of marine animals lacking backbones called invertebrates. Perhaps the best known of these creatures was the trilobite (Fig. 21),
Figure 21 Trilobite fossils of the Cambrian age
Carrara Formation in the
southern Great Basin of
California and Nevada.
(Photo by A. R. Palmer,
courtesy USGS)
28

41.
clues to the past
a primitive arthropod and ancestor of today’s horseshoe crab. They first
appeared at the base of the Paleozoic era, about 570 million years ago. The
trilobites became the dominant animals of the Paleozoic, diversifying into
some 10,000 species before declining and becoming extinct after some 340
million years of existence. Because trilobites were so widespread and lived for
so long, their fossils have become important markers (also called guide or
index fossils) for dating Paleozoic rocks.
The demise of the trilobites might be connected to the arrival of the
jawed fishes. Fish were among the first vertebrates, or animals with internal
skeletons.These provided more efficient muscle attachments, which gave fish
much better mobility than their invertebrate counterparts. Fish constitute
more than half the species of vertebrates, both living and extinct.The placoderms (Fig. 22) were fierce giants, growing 30 feet in length.They had thick
armor plating around the head that extended over and behind the jaws and
probably made them poor swimmers.They might have preyed on smaller fish,
which in turn fed on trilobites.
While fish were thriving in the ocean, plants advanced onto the land
beginning some 450 million years ago (Fig. 23).Within 90 million years, vast
forests covered the Earth. Their decay, burial, and metamorphism formed
many of today’s coal deposits (fossil fuel). Evolving along with the land plants
were the arthropods, which constitute the largest phylum of living organisms
and number roughly 1 million species, or about 80 percent of all known animals.These insects helped to pollinate the plants, whose flowers offered sweet
nectar in return for services rendered. Unfortunately, because of their delicate bodies, insects did not fossilize well. However, they could be preserved
if trapped in tree sap, which later hardened into amber, a clear yellow substance that allows the study of even the most minute body parts.
The vertebrates did not set foot on dry land until nearly 100 million
years after the plants appeared. The first to come ashore were the amphib-
Figure 22 The extinct
placoderms were heavily
armored and extended 30
feet in length.
29

42.
INTRODUCTION TO FOSSILS AND MINERALS
Figure 23 The emergence of plants from the
sea onto the land.
ians, which evolved into reptiles, which in turn gave rise to the dinosaurs.
Dinosaur bones are abundant in Jurassic and Cretaceous sediments in many
parts of the United States, particularly in the West. Alongside the dinosaurs
evolved the mammals, which for the most part were small nocturnal creatures that fed during the night so as not to compete directly with the
dinosaurs. The Cenozoic mammals are well represented in geologic history. Woolly mammoths (Fig. 24), extinct giant mammals of the late
Pleistocene ice age, have been well preserved in the deep freeze on top of
the world.
EVIDENCE FOR EVOLUTION
During a five-year period from 1831 to 1836, the British naturalist Charles
Darwin was employed as the ship’s geologist aboard the H.M.S. Beagle and
described in great detail the rocks and fossils he encountered on his journey
around the world (Fig. 25). Darwin was trained as a geologist and thought like
one, but today he tends to be viewed as a biologist. He made many significant
contributions to the field of geology, which during his day was entering its
golden age.
When Darwin visited the Galápagos Islands in the eastern Pacific, he
noticed major differences between plants and animals living on the islands and
their relatives on the adjacent South American continent. Animals such as
finches and iguanas assumed forms that were distinct from but related to those
of animals on adjacent islands. Cool ocean currents and volcanic rock made
the Galapagos a much different environment than Ecuador, the nearest land
600 miles to the east.The similarities among animals of the two regions could
30

43.
clues to the past
Figure 24 The woolly
mammoth went extinct at
the end of the last ice age.
only mean that Ecuadorian species colonized the islands and then diverged by
a natural process of evolution.
Darwin observed the relationships between animals on islands and on
adjacent continents as well as between animals and fossils of their extinct relatives. This study led him to conclude that species had been continuously
evolving throughout time. Actually, Darwin was not the first to make this
observation. His theory differed, however, in postulating that new parts
evolved in many tiny stages rather than in discrete jumps, which he attributed
to gaps in the geologic record caused by periods of erosion or nondeposition.
Figure 25 Darwin’s
journey around the world.
Starting out from Great
Britain, he sailed to South
America, Australia, Africa,
and back to Great
Britain.
Pacific
Ocean
Atlantic
Ocean
GALAPAGOS
ISLANDS
31

44.
INTRODUCTION TO FOSSILS AND MINERALS
Therefore, to Darwin, evolution worked at a constant tempo as species adapted to a constantly changing environment.
Darwin coined the phrase “survival of the fittest,” meaning that members of a particular species that can best utilize their environment have the best
chance of producing offspring that possess the survival characteristics of their
parents. In other words, successful parents have a better chance of passing on
their “good” genes to their offspring, which in turn are better able to survive
in their respective environment. Natural selection therefore favored those best
suited to their environment at the expense of less suited species. Contemporary
geologists embraced Darwin’s theory, for at last a clear understanding of the
changes in body forms in fossils of different ages was at hand.Thus, they could
place geologic events in their proper sequence by studying the evolutionary
changes that took place among fossils.
Evolution apparently was not always gradual and constant in tempo as
Darwin saw it.The fossil record implies that life evolved by fits and starts. Long
periods of little or no change were punctuated by short intervals of rapid
change and then followed by long periods of stasis (stay-as-is).The pattern of
change and stasis is called punctuated equilibrium. Species formed relatively
quickly as a result of rapid bursts of evolutionary change. New species evolved
within a few thousand years (practically instantaneously in geologic time) and
then remained essentially unchanged for up to several million years.
Species formed relatively quickly as a result of rapid bursts of evolutionary change. Furthermore, rapid evolutionary changes in large segments of
organisms might appear in the fossil record as having been caused by mass
extinction when in fact no actual extinction had occurred. Evolution also
might be opportunistic, with variations arising by chance and selected in
accordance with the demands of the environment. The evolution of a single
species also affects the evolution of others with which it interacts. When the
environment changes abruptly to one that is harsher, species incapable of
rapidly adapting to these new conditions cannot live at their optimum and
therefore do not pass on their “bad” genes to future generations.
Rapid evolutionary advancements might result from rare large mutations. Thus, evolution appears to make sudden leaps, with major changes
occurring simultaneously in many body parts. In other words, natural selection does not favor piecemeal tinkering; it therefore cannot work on structures that are not fully functional during intermediary periods of development
of new appendages. An example is the development of insect wings, which
were probably first used for cooling purposes. Later, as the benefits of flight
became apparent, the wing structures grew more aerodynamic, giving flying
insects an enormous advantage over their earth-bound competitors.
Evolutionary trends varied throughout geologic time in response to major
environmental changes, as natural selection acted to adapt organisms to the new
32

45.
clues to the past
conditions forced on them by several environmental factors, such as chemical
alterations in the ocean, climate changes, or mass extinctions. However, natural
selection is not deterministic. Variations are purely accidental and selected
according to the demands of the environment; the most adapted species have
the best chance of survival. Most of the time, species resist change, even though
the consequences make them better suited to environmental needs.
Gaps in the fossil record might result from the lack of intermediary
species, or so-called missing links, which apparently existed only in small populations. Small populations are less likely to leave a fossil record because the
process of fossilization favors large populations. Furthermore, the intermediates probably did not live in the same locality as their ancestors and thus were
unlikely to be preserved along with them. New species that start out in small
populations evolve rapidly as they radiate into new environments. Then as
populations increase, slower evolutionary changes take place as the species’
chances of entering the fossil record improve.
The fossil record also might suggest differences in fossil samples where
no actual differences exist. In any ecologic community, a few species occur in
abundance, some occur frequently, but most are rare and occur only infrequently. In addition, the odds of any individual’s becoming fossilized after
death, and thus entering the fossil record, are extremely small. No single fossil sample will contain all the rare species in an assemblage of species. If this
sample were compared with another higher up in the stratigraphic column,
which represents a later time in geologic history, an overlapping but different
set of rare species would be recorded. Species found in the lower sample but
not in the upper sample might erroneously be inferred to have gone extinct.
Conversely, species that appear in the upper sample but not in the lower sample might wrongly be thought to have originated there. Thus, the reading of
the fossil record often can be confusing and misleading.
A commonly held belief among scientists is that environmental change
drives evolution and not the other way around. However, the British chemist
James Lovelock turned the scientific community on its head in 1979 by
proposing the Gaia hypothesis, named for the Greek goddess of the Earth. He
postulated that the living world is able to control to some extent its own environment and that living organisms maintain the optimal conditions for life by
regulating the climate, similarly to the way the human body regulates its temperature to maintain optimal metabolic efficiency. The Gaia hypothesis also
suggests that from the very beginning, life followed a well organized pattern
of growth independent of chance and natural selection. Apparently, living
things kept pace with all the changes in the Earth over time and might have
made some major alterations of their own such as converting most of the carbon dioxide in the early atmosphere and ocean into oxygen through photosynthesis (Table 2).
33

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INTRODUCTION TO FOSSILS AND MINERALS
TABLE 2
Evolution
Origin of Earth
Origin of life
Photosynthesis
Eukaryotic cells
Sexual reproduction
Metazoans
Land plants
Land animals
Mammals
Humans
EVOLUTION OF LIFE AND THE ATMOSPHERE
Origin (million years)
4,600
3,800
2,300
1,400
1,100
700
400
350
200
2
Atmosphere
Hydrogen, helium
Nitrogen, methane, carbon dioxide
Nitrogen, carbon dioxide, oxygen
Nitrogen, carbon dioxide, oxygen
Nitrogen, oxygen, carbon dioxide
Nitrogen, oxygen
Nitrogen, oxygen
Nitrogen, oxygen
Nitrogen, oxygen
Nitrogen, oxygen
Perhaps the greatest forces affecting evolutionary changes were plate
tectonics and the drifting of the continents. Continental motions had a wideranging effect on the distribution, isolation, and evolution of species. The
changes in continental configuration greatly affected global temperatures, ocean
currents, productivity, and many other factors of fundamental importance to life.
The positioning of the continents with respect to each other and to the equator helped determine climatic conditions.When most of the land huddled near
the equatorial regions (Fig. 26), the climate was warm, but when lands wandered
into the polar regions the climate grew cold and brought periods of glaciation.
The changing shapes of the ocean basins due to the movement of continents affect the flow of ocean currents, the width of continental margins,
and, consequently, the abundance of marine habitats. When a supercontinent
breaks up, more continental margins are created, the land lowers, and the sea
level rises, providing a larger habitat area for marine organisms. During times
of highly active continental movements, the Earth experiences greater volcanic activity, especially at spreading centers, where tectonic plates are pulled
apart by upwelling magma from the upper mantle.The amount of volcanism
could affect the composition of the atmosphere, the rate of mountain building, the climate, and inevitably life itself.
MASS EXTINCTIONS
Practically all species that have ever existed on Earth are extinct (Table 3).
Throughout geologic history, species have come and gone on geologic time
34

47.
clues to the past
Figure 26 The approximate positions of the continents relative to the
equator during the
Devonian and
Carboniferous periods.
Eurasia
North
America
Eq
ua
to
r
Africa
South
America
scales, so that those living today represent only a tiny fraction of the total. Of
the 4 billion species of plants and animals thought to have existed in the geologic past, over 99 percent have become extinct. All extinction events appear
to indicate biologic systems in extreme stress brought on by a radical change
in the environment caused by large meteorite impacts or volcanic eruptions.
Most mass extinctions followed periods of environmental upheavals such as
global cooling.
The more devastating and globally encompassing an extinction event,
the greater the evolutionary change. For this reason, extinctions play an
enormous role in evolution. Extinction is therefore an inevitable part of the
evolutionary process and essential for the advancement of species.Therefore,
little happens in evolution without extinction’s first disrupting living conditions. Each mass extinction marks a watershed in the evolution of life, by
resetting the evolutionary clock, forcing species to start anew.When a major
extinction event occurs, new species develop to fill vacated habitats. Because
of their significant impacts on life, major extinction events also mark the
boundaries between geologic periods.
35

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INTRODUCTION TO FOSSILS AND MINERALS
TABLE 3
RADIATION AND EXTINCTION OF SPECIES
Organism
Radiation
Extinction
Mammals
Reptiles
Amphibians
Insects
Land plants
Fishes
Crinoids
Trilobites
Ammonoids
Nautiloids
Brachiopods
Graptolites
Foraminiferans
Paleocene
Permian
Pennsylvania
Upper Paleozoic
Devonian
Devonian
Ordovician
Cambrian
Devonian
Ordovician
Ordovician
Ordovician
Silurian
Pleistocene
Upper Cretaceous
Permian-Triassic
No major extinction
Permian
Pennsylvanian
Upper Permian
Carboniferous and Permian
Upper Cretaceous
Mississippian
Devonian and Carboniferous
Silurian and Devonian
Permian and Triassic
Marine invertebrates
Lower Paleozoic
Permian
The vast majority of the Earth’s fauna and flora lived during the
Phanerozoic eon from about 570 million years ago (mya) to the present (Fig.
27).This was a period of phenomenal growth as well as tragic episodes of mass
extinction, each involving the loss of more than half the species living at the
time. Five major extinctions, interspersed with five or more minor die outs,
occurred during this period. The first mass extinction occurred in the early
Cambrian (530 mya) and decimated over 80 percent of all marine animal genera; it was one of the worst in geologic history. A second mass dying at the
end of the Ordovician (440 mya) eliminated some 100 families of marine animals.Another major die off during the middle Devonian (365 mya) witnessed
the mass disappearance of many tropical marine groups.
The greatest loss of life in the fossil record occurred at the end of the
Permian (250 mya), when half the families of organisms comprising more
than 95 percent of all marine species and 80 percent of all terrestrial species,
disappeared. Another tragic event at the end of the Triassic (210 mya) took
the lives of nearly half the reptilian species.The most familiar die out eliminated 70 percent of all known species including the dinosaurs (Fig. 28) at
the end of the Cretaceous (65 mya). All extinction events seem to indicate
biologic systems in extreme stress brought on by climate change or a drop
in sea level.
36

49.
clues to the past
Figure 27 The number
of families through time.
Note the large dip during
the Permian extinction
Quaternary
2
Pliocene
Miocene
Million years before present
Oligocene
Eocene
Paleocene
65
Cretaceous
Jurassic
Triassic
230
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
570
0
50
100
150
200
250
300
Number of families
The fossil record suggests that mass extinctions might be somewhat periodic, possibly resulting from celestial influences such as cosmic rays from
supernovas or huge meteorite impacts.Ten or more large asteroids or comets
have collided with the Earth over the last 600 million years. Analysis of 13
major impact craters distributed over a period from 250 million to 5 million
years ago suggests a rate of one crater roughly every 28 million years.
Since the great Permian catastrophe, eight significant extinction events,
which defined the boundaries of the geologic time scale, have occurred; many
of the strongest peaks have coincided with the boundaries between geologic
periods.The episodes of extinction appear to be cyclical, occurring every 26
to 32 million years. Longer intervals of 80 to 90 million years are related to
the breakup and collisions of continents. Exceptionally strong extinctions
occur every 225 to 275 million years, corresponding to the solar system’s rotational period around the center of the Milky Way galaxy.
The extinctions might merely be episodic, with relatively long periods
of stability followed by random, short-lived (geologically speaking) extinction
events that only appear periodic. Major extinctions therefore could reflect a
37

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INTRODUCTION TO FOSSILS AND MINERALS
Figure 28 All species of
dinosaurs went extinct at
the end of the Cretaceous
as a result of a number of
suspected causes, including
terrestrial as well as
extraterrestrial influences.
38
clustering of several minor events at certain times that, because of the nature
of the fossil record, only mimics a cyclical pattern. In other words, random
groupings of extinct species on a geologic time scale that is itself uncertain
could simply be coincidental. Furthermore, a short period of rapid evolution
might manifest itself in the geologic record as though preceded by a mass
extinction when indeed none had occurred.
Major extinctions are separated by periods of lower extinction rates, called
background extinctions, and the difference between them is only a matter of
degree. Therefore, mass extinctions are not simply intensifications of processes
operating during background times. Species have regularly come and gone even
during optimal conditions.Those that suffer extinction might have been developing certain unfavorable traits. Extinct species also might have lost their competitive edge and been replaced by a superior, more adaptable species. Certain
characteristics that permit a species to live successfully during normal periods

51.
clues to the past
for some reason become irrelevant when major extinction events occur.Thus,
the extinct dinosaurs might not have done anything “wrong” biologically.
The distinction between background and mass extinctions might be distorted by ambiguities in the fossil record, especially when certain species are
favored over others for fossilization. Only under demanding geologic conditions that promote rapid burial with little predation or decomposition are the
bodies of dead organisms preserved to withstand the rigors of time. Because
species with hard body parts fossilize better than soft-bodied organisms, they
are more likely to be represented in the fossil record and therefore present a
skewed account of historic geology.
Catastrophic extinction events appear to be virtually instantaneous in
the fossil record because discerning a period of several thousand years over
millions of years of geologic time is not possible. More likely, the extinctions
occurred over lengthy periods of perhaps a million years or more, and because
of erosion or nondeposition of the sedimentary strata that preserve species as
fossils the die outs only appear sudden. Several times in Earth history, sea levels have fallen, reducing sedimentation rates and the preservation of species.
Therefore, a sudden break in geologic time might in reality have extended
over a lengthy period.
Those species that survive mass extinction radiate outward to fill new
environments, which in turn produce entirely new species.These might develop novel adaptations that give them a survival advantage over other species.
The adaptations might lead to exotic-looking species that prosper during normal background times, but because of their overspecialization are incapable of
surviving mass extinctions. Therefore, the fossil record shows myriad strange
creatures, the likes of which have never been seen since (Fig. 29).
Figure 29 Helicoplacus,
an experimental species
with body parts assembled
differently from those of
every other living creature,
became extinct about 510
million years ago after
surviving for 20 million
years.
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INTRODUCTION TO FOSSILS AND MINERALS
The geologic record implies that nature is constantly experimenting
with new forms of life, and when one fails, such as the dinosaurs, it becomes
extinct, and the odds against the reappearance of its particular combination of
genes are astronomical.Thus, evolution seems to run on a one-way track, and
although it perfects species to live at their optimum in their respective environments, it can never go back to the past.That is why even though the environment of the future might match that of the Cretaceous period when the
dinosaurs roamed the Earth, they will never return.
Dinosaurs were not the only ones to go, and 70 percent of all known
species vanished at the end of the Cretaceous. Therefore, something in the
environment made them all unfit to survive, yet this factor did not significantly affect the mammals. Dinosaurs and mammals coexisted for more than
100 million years. After the dinosaurs became extinct, the mammals underwent an explosive evolutionary radiation, which gave rise to many unusual
species, some of which became extinct early in the Cenozoic.
Many geologists are beginning to accept catastrophe as a normal occurrence in Earth history and as a part of the uniformitarian process, also called
gradualism. Certain periods of mass extinctions appear to be the result of some
catastrophic event, such as the bombardment of one or more large asteroids or
comets, rather than subtle changes, such as a change in climate or sea level or
an increase in predation.Therefore, mass extinctions appear to be part of a pattern of life throughout the Phanerozoic.
GEOLOGIC AGE DATING
Both large and small extinctions were used by 19th-century geologists to
define the boundaries of the geologic time scale (Table 4). But because no
means was available to date rocks, the entire geologic record was delineated
by using relative dating techniques, which only indicated which bed was older
or younger in accordance to its fossil content. Therefore, relative dating only
places rocks in their proper sequence but does not indicate how long ago an
event took place, only that it followed one event and preceded another. Before
the development of radiometric dating techniques, geologists had no method
of dating events precisely. So relative dating techniques were developed, and
they are still in use today.Absolute dating methods did not replace these techniques, however, but only supplemented them.
The problem with assigning absolute dates to units of relative time is that
most radioactive isotopes are restricted to igneous rocks. Even if sedimentary
rocks, which constitute most of the rocks on the Earth’s surface and contain
practically all the fossils, did possess a radioactive mineral, most rocks could not
be dated accurately because the sediments were composed of grains derived
40

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INTRODUCTION TO FOSSILS AND MINERALS
from older rocks.Therefore, in order to date sedimentary rocks, geologists had
to relate them to igneous masses. A layer of volcanic ash deposited above or
below a sedimentary bed could be dated radiometrically, as could crosscutting features such as granitic dikes which are always younger than the beds
they cross.The sedimentary strata would then be bracketed by dated materials, and the age could be estimated.
The radiometric dating method measures the ratios of radioactive parent materials to their daughter products and compares this ratio to the known
half-lives of the radioactive elements. The half-life is the time required for
one-half of a radioactive element to decay to a stable daughter product. For
example, if one pound of a hypothetical radioactive element had a half-life of
1 million years, then after a period of 1 million years a half-pound of the original parent material and a half-pound of daughter product would be present.
The ratio of parent element to its daughter product is determined by chemical and radiometric analysis of the sample rock.Therefore, if the quantities of
parent and daughter are equal, one half-life has expired, making the sample 1
million years old.After 2 million years, one-quarter of the original parent element remains in the sample, and after 4 million years only one-sixteenth is
left. Generally, radioactive elements are usable for age dating up to about 10
half-lives. Afterward, the amount of parent material is reduced to about a
thousandth of its original mass.
Radioactive decay also appears to be constant with time and is unaffected by chemical reactions, temperature, pressure, or any other known conditions or processes that could change the decay rate throughout geologic
history. Confirmation that decay rates are steady throughout time is found in
certain minerals such as biotite mica. Extremely small zones of discoloration,
or halos, are found surrounding minute inclusions of radioactive particles
within the crystal.The haloes consist of a series of concentric rings around the
radioactive source. Particles emitted by the radioactive source damage the surrounding biotite minerals. The energy of the particle is determined by the
distance it travels through the mineral and depends on the type of radioactive
element responsible. Since the radii of concentric rings corresponds to the
energy of present-day particles, particle energies have not changed, and therefore the rate of radioactive decay remains constant over time.
The precision of radiometric age dating depends on the accuracy of the
chemical and radiometric analyses that determine the amount of the parent
element and daughter product; it also depends on whether either has been
added to or removed from the sample since deposition.The quantities of these
substances might only be on the order of a few parts per million of the rock
mass. A certain amount of naturally occurring daughter material might have
existed in the rock before the parent element began decaying. Moreover, many
radioactive elements do not decay directly into stable daughter products but
42

55.
clues to the past
go through a series of intermediate decay schemes, further complicating
analysis.
Of all the radioactive isotopes that exist in nature, only a few have been
proved useful in dating rocks. All others either are very rare or have half-lives
that are too short or too long. Rubidium-87 with a half-life of 47 billion
years, whose daughter product is strontium-87, is useful for dating rocks older
than 20 million years. Uranium-238 with a half-life of 4.5 billion years, and
uranium-235 with a half-life of 0.7 billion years, whose daughter products are
lead-206 and lead-207, respectively, are useful for dating rocks more than 100
million years old.The uranium isotopes are important for dating igneous and
metamorphic rocks. Because both species of uranium occur together, they also
can be used to cross-check each other.
Potassium-40 is more versatile for dating younger rocks. Although the
half-life of potassium-40 is 1.3 billion years, recent analytical techniques allow
the detection of minute amounts of its stable daughter product argon-40 in
rocks as young as 30,000 years old. It is less precise for dating younger rocks
because of the relatively small amount of daughter product available in the
sample. Minerals such as hornblende, nepheline, biotite, and muscovite are
used for dating most igneous and metamorphic rocks by the potassium-argon
method.
Sedimentary rocks present a more difficult problem for radiometric dating because their material was derived from weathering processes. Fortunately,
a micalike mineral called glauconite forms in the sedimentary environment
and contains both potassium-40 and rubidium-87. As a result, the age of the
sedimentary deposit can be established directly by determining the age of the
glauconite. Unfortunately, metamorphism, no matter how slight, might reset
the radiometric clock by moving the parent and daughter products elsewhere
in the sample. In this case, the radiometric measurement can only date the
metamorphic event. In order to date these rocks accurately, a whole-rock
analysis must be made, using large chunks of rock instead of individual crystals. Sediments also can be dated by using optically stimulated thermoluminescence, which measures when sand grains were last exposed to light and is
especially useful for dating fossil footprints.
To date more recent events, the radioactive isotope carbon-14, or radiocarbon, is used. Carbon-14 is continuously created in the upper atmosphere
by cosmic ray bombardment of gases, which in turn release neutrons. The
neutrons bombard nitrogen in the air, causing the nucleus to emit a proton,
thus converting nitrogen into radioactive carbon-14. In chemical reactions,
this isotope behaves identically to natural carbon-12. It reacts with oxygen to
form carbon dioxide, circulates in the atmosphere, and is absorbed directly or
indirectly by living matter (Fig. 30). As a result, all organisms contain a small
amount of carbon-14 in their bodies. Carbon-14 decays at a steady rate with
43

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INTRODUCTION TO FOSSILS AND MINERALS
Figure 30 The carbon14 cycle. Cosmic rays
striking the atmosphere
release neutrons that strike
nitrogen atoms to produce
carbon-14, which is converted into carbon dioxide
and taken in by plants
and animals.
Cosmic rays
Atmosphere
Neutron
N
C-14
O
Proton
O
CO2
C-14 decay
a half-life of 5,730 years.When an organism is alive, the decaying radiocarbon
is continuously being replaced, and the proportions of carbon-14 and carbon12 remain constant. However, when a plant or animal dies, it ceases to take in
carbon and the amount of carbon-14 gradually decreases as it decays to stable
nitrogen-14. This results from the emission of a beta particle (free electron)
from the carbon-14 nucleus, thus transmuting a neutron into a proton and
restoring the nitrogen atom to its original state.
Radiocarbon dates are determined by chemical analysis, which compares
the proportion of carbon-14 to that of carbon-12 in a sample (Fig. 31). The
development of improved analytical techniques has increased the usefulness of
radiocarbon dating, and it can be used to date events taking place more than
100,000 years ago. Furthermore, paleontologists, anthropologists, archaeologists, and historians now have a means of accurately dating events from our
distant past.
44

57.
clues to the past
Figure 31 Scientist dating a sample by the radiocarbon method.
(Courtesy USGS)
THE GEOLOGIC TIME SCALE
Geologists measure geologic time by tracing fossils through the rock strata and
noticing the greater change in the deeper rocks than in those near the surface.
Fossil-bearing strata can be followed horizontally over great distances, because
a particular fossil bed can be identified in another locality with respect to beds
above and below it.These are called marker beds and are used for identifying
geologic formations and for delineating rock units for geologic mapping.
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INTRODUCTION TO FOSSILS AND MINERALS
When fossils are arranged according to their age, they do not present a
random or haphazard picture, but instead show progressive changes from simple to complex forms and reveal the advancement of the species through time.
Geologists are thus able to recognize geologic time periods on the basis of
groups of organisms that were especially plentiful and characteristic during a
particular time.Within each period, many subdivisions are determined by the
occurrence of certain species, and this same succession is found on every
major continent and is never out of order.
Fossils are necessary for correlating rock units over vast distances because
the lithology of rock strata changes from one location to another. A major
problem is that few fossil-bearing formations have survived undisturbed over
time. Since certain species have existed only during specific intervals, their
respective fossils can place stratigraphic units in their proper sequence, which
defines the relative time periods. These beds can then be traced over wide
areas by comparing their fossil content, providing a comprehensive geologic
history over a broad region.The use of fossils has established a geologic time
scale that can be applied to all parts of the world.
Although the existence of fossils had been known since the early Greeks,
their significance as a geologic tool was not discovered until the late 18th century. In the 1790s, the English civil engineer William Smith found that rock
formations in the canals he built across Great Britain contained fossils significantly different from those in the beds above or below. He noted that layers
from two different sites could be regarded as equivalent in age as long as they
contained the same fossils. Therefore, sedimentary strata in widely separated
areas could be identified by their distinctive fossil content. Furthermore, one
type of bed such as sandstone might grade into a different bed such as limestone that contained the same fossils, indicating they were the same age.
Using the characteristics of the different strata and their fossils, Smith
drew geologic maps of the varied rock formations throughout Britain. He
made the most significant contribution to the understanding of fossils when
he proposed the law of faunal succession, which stated that rocks could be
placed in their proper time sequence by studying their fossil content.This law
became the basis for the establishment of the geologic time scale and the
beginning of modern geology.
The French geologists Georges Cuvier and Alexandre Brongniart
refined this approach with their discovery that certain fossils in rocks around
Paris were confined to specific beds. The geologists arranged fossils in a
chronological order and noticed a systematic variation according to their positions in the geologic column. Fossils in the higher rock layers more closely
resembled modern species than those farther down.The fossils did not occur
randomly but in a determinable order from simple to complex. Units of geologic time could thus be identified by their distinctive fossil content.
46

59.
clues to the past
In 1830, the British geologist Charles Lyell took these ideas one step further by proposing that rock formations and other geologic features took
shape, eroded, and re-formed at a constant rate throughout time according to
the principle of uniformitarianism—the concept that the present is the key to
the past. In other words, the forces that shaped the Earth are uniform and
operated in the past much as they do today.The theory was originally developed in 1785 by Lyell’s mentor, the Scottish geologist James Hutton, known
today as the “father of geology.”
The history of the Earth has been divided into units of geologic time
according to the type and abundance of fossils present in the strata.The periods take their names from the localities with the best exposures (Fig. 32).
For example, the Jurassic period is named for the Jura Mountains in
Switzerland, whose limestones provide a suite of fossils that adequately
depicts the period.
Stratigraphic units are classified into erathems, consisting of the rocks
formed during an era of geologic time. Erathems are divided into systems,
consisting of rocks formed during a period of geologic time. Systems are
divided into groups, consisting of rocks of two or more formations that contain common features. Formations are classified by distinctive features in the
rock and are given the name of the locality where they were originally
ea
North
Sea
B al
5
1
2
3
4
Atlantic
Ocean
ti c
S
Figure 32 Type locations
for geologic periods:
(1) Cambrian,
(2) Ordovician,
(3) Silurian,
(4) Devonian,
(5) Carboniferous,
(6) Triassic, (7) Jurassic,
(8) Cretaceous, (9) Tertiary,
(10) Quaternary.
6
10
8
7
9
Mediterranean Sea
47